The present invention relates to video processing, and more particularly to circuit and method for adjusting the sharpness of edges in video signals.
Color video information is often transmitted and stored as a composite (or CVBS) signal that includes a luminance component (Y), a chrominance component (C), a blanking signal, and vertical and horizontal synchronization signals. The luminance component expresses the intensity (i.e., black to white) of a picture, and the chrominance component expresses color and its intensity. The composite signal is typically decoded into a luminance signal and color difference signals (e.g., U and V, or I and Q) prior to display on a television or a monitor.
A video picture can appear more sharp by properly processing the edges in the luminance signal. Specifically, by detecting black to white and white to black transitions in the picture, and increasing the rate or slope of the transitions, the picture will appear more sharp to the viewers. A sharper picture can provide more details, and is desirable in many applications.
Conventionally, sharpness enhancement of edges is performed by filtering a luminance signal with a filter (e.g., a bandpass filter), scaling the filtered signal with a gain factor, and summing the scaled signal with the luminance signal to generate an output signal having sharper edges. The filter is used to extract higher frequency components that are typically indicative of edges in the luminance signal. This technique for increasing sharpness is disclosed in U.S. Pat. No. 5,790,205, which is incorporated herein by reference.
Yet another technique for sharpening rising and falling edges of video signals is disclosed in U.S. Pat. No. 5,491,520, which is also incorporated herein by reference. In this technique, the luminance signal is differentiated to generate intermediate signals that are indicative of rising and falling edges in the video signals. The intermediate signals are further processed (e.g., scaled and limited) and summed with the luminance signal to provide an output signal having sharper edges.
These conventional techniques generally provide adequate performance by xe2x80x9clinearlyxe2x80x9d processing the luminance signal. In the linear process, a particular set of operations (e.g., filtering, scaling, and so on) is applied across the entire edge, thus making it a challenge to enhance the center of the edges to a great extent without introducing excessive amounts of undershoot and overshoot at the boundaries of the enhanced edges.
Thus, techniques that can provide more enhanced edges in video signals while reducing the amount of undershoots and overshoots are highly desirable.
The invention provides techniques for enhancing edges in video signals while reducing the amount of undershoots and overshoots. A video signal is processed to generate a first signal that indicates detected edges in the video signal. The first signal can be generated, for example, by lowpass filtering the video signal to generate a lowpass signal and subtracting the lowpass signal from a luminance signal that has been extracted from the video signal. The first signal is then processed with a xe2x80x9cnon-linearxe2x80x9d transfer function, as described below, to generate a second signal having enhanced edges. The second signal is used as the correction or enhancement signal, and is added to the lowpass signal to provide an output signal having enhanced edges with reduced or minimal amounts of undershoots and overshoots.
An embodiment of the invention provides a circuit for enhancing edges in a video signal. The circuit includes a luminance filter, an edge enhancement circuit, and a combiner. The luminance filter receives the video signal and provides a lowpass signal and a first signal that is indicative of detected edges in the video signal. The edge enhancement circuit receives the first signal and provides a second signal that is a non-linear function of the first signal. The combiner receives and combines the lowpass and second signals to provide an output signal having enhanced edges.
The second signal has one or more of the following characteristics: (1) it is dynamically generated based on characteristics of the detected edges in the video signal; (2) it provides varying amounts of enhancement across the detected edges in the video signal; (3) it provides higher amounts of enhancement near the center of the detected edges and smaller amounts of enhancement away from the center; and (4) it provides an amount of enhancement that is dependent on the slope of the detected edges.
The non-linear function can be implemented by processing the first signal to generate a third signal, and to multiply the first signal with the third signal to generate the second signal. In a specific implementation, the non-linear function is achieved by a combination of the following: (1) highpass or bandpass filtering of the first signal; (2) taking the absolute values of the filtered signal; (3) scaling the absolute values with a gain factor; (4) limiting the scaled signal; (5) offsetting the limited signal with an offset factor; (6) coring a version of the first signal; and (7) multiplying the cored signal with the offsetted signal.
The luminance filter can include a two-dimensional filter that can be implemented as a finite impulse response (FIR) filter having an Mxc3x97N transfer function. The two-dimensional filter can be implemented as a decomposable filter composed of a vertical filter followed by a horizontal filter. The two-dimensional filter can be designed to average three or more (e.g., five) samples in the horizontal direction and two or more video lines in the vertical direction. The video lines can be obtained by delaying the video signal with delay elements having variable delays. These delay elements can also be used to implement a chrominance filter, which is used to extract the chrominance component from the video signal.
Another embodiment of the invention provides a Y/C separator circuit that includes at least one delay element, a luminance filter, an edge enhancement circuit, a combiner, and one or more adder circuits. The delay element(s) receive and delay a video signal, with each delay element providing approximately one horizontal line of delay. The luminance filter receives the video signal and zero or more delayed signals, filters the received signal(s) to provide a lowpass signal, and generates a first signal indicative of detected edges in the video signal. The edge enhancement circuit receives the first signal and provides a second signal that is a non-linear function of the first signal. The combiner receives and combines the lowpass and second signals to provide an output luminance signal having enhanced edges. The adder circuit(s) receive the video signal and one or more delayed signals, combines the received signals, and provides a chrominance signal. The luminance filter and edge enhancement circuit can be designed in similar manners as that described above.
Yet another embodiment of the invention provides a video decoder for decoding a composite video signal. The video decoder includes an input resampler coupled to a Y/C separator. The input resampler receives and resamples input video samples with a first resampling signal to generate resampled video samples that define a resampled signal. The Y/C separator receives and separates the resampled signal into a luminance signal and a chrominance signal. The Y/C separator includes a luminance filter, an edge enhancement circuit, a combiner, and a chrominance filter. The elements of the Y/C separator can be implemented in similar manners as that described above.
Yet another embodiment of the invention provides a method for generating enhanced edges in a video signal. In accordance with the method, the video signal is initially filtered to provide a lowpass signal that is then combined with a luminance signal that has been extracted from the video signal to provide a first signal indicative of detected edges in the video signal. The first signal is processed with a non-linear function to provide a second signal indicative of enhanced edges in the video signal. The lowpass and second signals are combined to provide an output signal having enhanced edges. The second signal has one or more of the characteristics enumerated above.
The non-linear function can be achieved by processing the first signal and combining the processed first signal with a version of the first signal. In a specific implementation, the non-linear function can be achieved by a combination of the following: (1) filtering the first signal to remove low frequency components; (2) taking absolute values of the filtered first signal; (3) scaling the absolute values with a gain factor; (4) limiting the scaled signal; (5) offsetting the limited signal with an offset factor; (6) coring a version of the first signal to remove high frequency noise; and (7) multiplying the cored signal with the offsetted signal to generate the second signal.
The foregoing, together with other aspects of this invention, will become more apparent when refining to the following specification, claims, and accompanying drawings.